Infection dynamics of digital dermatitis in first-lactation Holstein cows in an infected herd.

Digital dermatitis (DD) refers to painful lesions primarily affecting the skin in the interdigital region of dairy cattle. The purpose of this study was to evaluate the dynamics of DD in 39 cows, observed at approximately 3-d intervals, for the first 6 mo of lactation. Specifically, the study aimed at evaluating different levels of DD susceptibility in cows, identifying the bacterial colonization of the interdigital skin, and exploring the relationship between clinical DD diagnosis and laboratory findings. Three different susceptibility categories were identified for DD: 1=consistently healthy cow; 2=intermittently infected cow; and 3=consistently infected cow. Susceptibility categories were associated with age at calving. The average age at calving was 775 d (SD ±43.4), with the youngest heifer calving at age 669 d and the oldest heifer at 858 d. Advancing age at calving was associated with greater odds of being intermittently or consistently infected. This corresponded with an odds ratio of 2.02 over a period of 30 d. During the study period, 161 DD lesions were identified in 28 of the 39 cows (72%). Of those 28 cows, 13 cows were consistently infected. The remaining 11 of the 39 cows (28%) showed slight thickening of the skin with no pain (5 cows) and no signs of skin changes (6 cows). Histopathology and fluorescence in situ hybridization were possible to perform on 132 biopsy samples. A clinical diagnosis of DD was confirmed in 70% of the lesions by histopathology, and colonization of Treponema spp. Dichelobacter nodosus was found in 35 samples (29%).